How to Find the Serial Number of Your Apple Computer ~ System Help Line

From the menu bar, click on the Apple logo and then click on About This Mac
The About This Mac window will open, click on the OS X Version number
The first click will display to you the Mac Build number
Click once again, and the serial number will be displayed. However, if you want to copy this serial number, you will not be able to select and copy it.

If you clicked once again, the MAC OS X version number will be displayed again
If you want to copy this serial number, then click on More Info...

Under the Overview section, the Serial Number is displayed and in this window you can select and copy the Serial number.

Another place where you can copy the serial number is by clicking on the System Report... button

and then the Serial Number will be displayed where you can copy it from this window as well.